This invention relates to selective call radio communication receivers such as pocket bells and is intended to provide a novel power supply system therefor.
For radio receivers of this kind, a digital transmission system which employs digital channel signals as formed by FSK (Frequency Shift Keying) modulation is usable in place of the conventional tone system, for which reference is made, for example, to the U.S. Pat. No. 3,651,413, issued Mar. 21, 1972, to K. H. Wycoff.
Receivers usable with such transmission system are constituted principally of a radio receiver circuit, a decoder circuit, a low-frequency amplifier circuit, a loudspeaker, a clock oscillator and a battery. The FSK signal input received through an antenna is demodulated into the radio receiver circuit and directed therefrom to the decoder circuit. The decoder circuit, being fed with the demodulated signal, decides whether the selecting signal for the receiver station has been received or not and, upon deciding that such signal has been received, acts to drive the low frequency amplifier. The output of the amplifier is directed to the loudspeaker, thus causing the latter to produce a call sound. As is generally known, the oscillator circuit includes such components as a resistor, an amplifier, a capacitor and a quartz oscillator and outputs clock pulses to the decoder circuit, which produces different timing signals required for receiver operation by frequency-dividing the clock output of the oscillator circuit. The frequency of the call sound can also be formed by frequency division of the clock. The battery is connected through a source switch with the receiver components, including the oscillator circuit. With this conventional arrangement, the oscillator circuit is inoperative as long as the power supply switch is open and starts oscillation only when the switch is closed.
In this connection, where the oscillator unit used is a quartz oscillator of the order of 30 KHz, having an equivalent resistance as high as several tens of kiloohm, it takes several seconds following application of power for the oscillations to attain the normal oscillating frequency. On the other hand, a selective call radio receiver such as a paging receiver is generally designed so as to enable the user to test its operation characteristics by closing the power supply switch thereby to produce a call sound. With radio receivers of the latter kind, however, the lapse of substantial time between the switch closing and the actual start of the call sound is a serious disadvantage. In this connection, another disadvantage is that until the normal frequency is reached the sound produced is unusual and offensive to the ear. Further, recent receiver models include a single cell as a power supply source with a source voltage on the order of 1.0 V to 1.5 V. On the other hand, the voltage required to render a crystal oscillator circuit operative is higher than the voltage at which the oscillator, once operating, is rendered inoperative, and often amounts to about 1.3 V. This apparently means that with such conventional receiver the battery becomes useless before its voltage falls down to the level at which oscillation would otherwise cease. In other words, such receiver has the further disadvantage that the service life of the battery in effect expires when its voltage is still at a relatively high level.